User Agent and Assistive Technology Support Notes

Description

The objective of this technique is to provide text alternatives for
images via an /Alt entry in the property list for a Tag. This is normally
accomplished using a tool for authoring PDF.

PDF documents may be enhanced by providing alternative descriptions
for images, formulas, and other items that do not translate naturally
into text. In fact, such text alternatives are required for accessibility:
alternate descriptions are human-readable text that can be vocalized
by text-to-speech technology for the benefit of users with vision disabilities.

When an image contains words that are important to understanding the
content, the text alternative should include those words. This will
allow the alternative to accurately represent the image. Note that
it does not necessarily describe the visual characteristics of the
image itself but must convey the same meaning as the image.

Example 5: Adding a text alternative to an image in a PDF document
using an /Alt entry

The /Alt property used on an image of mountains with a moon and trees
typically would be used like this (typically accomplished by an authoring
tool):

/Figure <</Alt (Sketch of Mountains with moon rising over trees) >>

The image might also be represented by a tag with a different name.
A different name might be used because the tag name is written in
a language other than English or because a specific tool uses a different
name for some other reason. In this situation, it is also necessary
that the RoleMap contained within the StructTreeRoot for the PDF
document contain an entry which explicitly maps the name of the tag
used for the image with the standard structure type used in PDF documents
(in this case, Figure). If the RoleMap contains only an entry mapping
Shape tags to Figure tags, the rolemap information would appear as
follows:

/RoleMap << /Shape /Figure >>

In this case, the usage of the /Alt entry as follows would also
be correct:

/Shape <</Alt (Crater Lake in the summer, with the blue sky, clouds and crater walls perfectly reflected in the lake) >>

Note that the /Alt entry in property lists can be combined with
other entries.

Related Techniques

Tests

Procedure

Verify the images which need equivalents have /Alt entries on
an enclosing tag by one of the following:

Read the PDF document with a screen reader, listening to hear
that the equivalent text is read when tabbing to the non-text
object (if it is tabbable) or hearing the alternative text read
when reading the content line-by-line.

Using a PDF editor, check that a text alternative is displayed for each
image.

Use a tool which is capable of showing the /Alt entry value,
such as aDesigner, to open the PDF document and view the GUI
summary to read the text alternatives for images.

Use a tool that exposes the document through the accessibility
API and verify that images have required text equivalents.

Expected Results

Check 1 is true for each image in the document which needs a text
equivalent.

If this is a sufficient technique for a success criterion, failing this test procedure does not necessarily mean that the success criterion has not been satisfied in some other way, only that this technique has not been successfully implemented and can not be used to claim conformance.